Organization Of Hydrocarbon Chains Research Articles

All-atom molecular dynamics simulation studies of fully hydrated gel phase DPPG and DPPE bilayers

Here in silico lipid membranes are described providing a structural background of the organization of the lipid components of membranes and aiding further biological or biophysical studies. An all-atom molecular dynamics simulations has been performed to investigate structural and dynamical properties of two fully hydrated gel-phase bilayers of 1,2-dipalmitoyl- sn-glycero-3-phosphoglycerol (DPPG) and 1,2-dipalmitoyl- sn-glycero-3-phospho-ethanolamine (DPPE) bilayers at 303 K. The respective starting configuration of lipids in the simulation bilayer unit cells were taken on the basis of scattering data. In both simulations, we found overall reasonably good agreement with the available experimental data (area per lipid, phosphorus–phosphorus distance). The distribution of the water/counterions at the membrane interface, interactions/orientations of lipid headgroups, and hydrocarbon chain organization were extensively studied in terms of pair distribution functions between main structural components of the system. Intra/intermolecular hydrogen bond formation was discussed in detail. The water orientation at the lipid membrane interface was explored thoroughly in terms of dipole moment as a function of the water molecule positions along the membrane, where we found that the counterions changed the orientation of the water at the interface. Special attention has been devoted to the distribution of the sodium counterions around the DPPG headgroup. We found preferential binding of Na + ions to the phosphate oxygen species.

Influence of cholesterol on the bilayer properties of monounsaturated phosphatidylcholine unilamellar vesicles

The influence of cholesterol on the structure of unilamellar-vesicle (ULV) phospholipid bilayers is studied using small-angle neutron scattering. ULVs made up of short-, mid- and long-chain monounsaturated phospholipids (diCn :1PC, n = 14 , 18, 22, respectively) are examined over a range (0-45 mol %) of cholesterol concentrations. Cholesterol's effect on bilayer structure is characterized through changes to the lipid's transmembrane thickness, lateral area and headgroup hydration. For all three lipids, analysis of the experimental data shows that the addition of cholesterol results in a monotonic increase of these parameters. In the case of the short- and mid-chain lipids, this is an expected result, however, such a finding was unexpected for the long-chain lipid. This implies that cholesterol has a pronounced effect on the lipid's hydrocarbon chain organization.

Liposome Structure Imaging by Atomic Force Microscopy: Verification of Improved Liposome Stability during Adsorption of Multiple Aggregated Vesicles

Atomic force microscopy has enabled direct visualization of the liposome structure supported on mica surfaces in air and silanized mica surfaces in aqueous media. The images display distinct patterns of adhered liposomes: multiple and single vesicle liposomes and flat supported bilayers. The multiple vesicle liposome structure is not visible by optical microscopy since the vesicles forming the liposome have diameters as small as 20 nm. Molecularly resolved force versus distance curves displaying the organization of hydrocarbon chains (mono- or bilayers) corroborate the presence of distinct adsorbed structures observed by scanning the surface. The high resolution of the observed liposome images allows the visualization of the aggregation of the multiple vesicles forming liposomes which were shown to have their origin in the liposome formation process and not during adsorption. Since most of the observed liposomes are aggregated vesicles, this aggregated structure has a substantially larger stability during adsorption than the single vesicle structure and consequently a larger resistance in maintaining its shape and function as a carrier of cosmetics, food additives, and drugs. This observation also has some important consequences in the liposomes' selective permeability when they are used as carriers.

Conformation and orientation of alkyl chains in the layered organic–inorganic hybrids: (CnH2n+1NH3)2PbI4(n = 12,16,18)

Layered organic–inorganic hybrids based on perovskite-derived alkylammonium lead halides have been demonstrated as important new materials in the construction of molecular electronic devices. Typical of this class of materials are the single-perovskite slab lead iodides of the general formula (CnH2n+1NH3)2PbI4. While for small n, these compounds are amenable to single-crystal structure determination, the increasing degree of disorder in the long chain (n = 12,14… ) compounds makes such an analysis difficult. In this study, we use powder X-ray diffraction, and vibrational and 13C NMR spectroscopies to establish the conformation, orientation and organization of hydrocarbon chains in the series of layered alkylammonium lead iodides (CnH2n+1NH3)2PbI4 (n = 12,16,18). We find that the alkyl chains adopt a tilted bilayer arrangement, while the structure of the inorganic layer remains invariant with respect to the value of n. Conformation-sensitive methylene stretching modes in the infrared and Raman spectra, as well as the 13C NMR spectra indicate that bonds in the methylene chain are in trans configuration. The skeletal modes of the alkyl chain in the Raman spectra establish that there is a high degree of all-trans conformational registry for the values of n studied here. From the orientation dependence of the infrared spectra of crystals of (CnH2n+1NH3)2PbI4 (n = 12,16), we find that the molecular axis of the all-trans alkyl chains are tilted away from the interlayer normal by an angle of 55°. This value of this tilt angle is consistent with the dependence of the c lattice expansion as a function of n, as determined from powder X-ray diffraction.

Laser Raman spectroscopic study of specifically deuterated phospholipid bilayers.

Raman spectra of 1,2-dimyristoyl-sn-glycero-3-phosphocholines specifically deuterated in the 2 chain at one of positions 3, 4, 6, 10, 12, and 14 have been obtained as a function of temperature. The frequencies of the CD2 vibrational stretching modes depend on the position of the labeled CD2 group, being maximum at position 3 of the acyl chain and then decreasing until they become constant beyond position 6. This frequency dependence is interpreted in terms of the inductive effect of the charge distribution of the acyl chain carboxyl group. In both gel and liquid-crystal phases, the Raman line widths depend on the position of the CD2 group, being minimum at position 6 and increasing toward both ends of the hydrocarbon chain. The width of the CD stretching bands abruptly increases at the phase transition temperature. The magnitude of the increase depends upon the position of the label, increasing almost linearly up to position 10 and then decreasing for positions 12 and 14. The spectra for the CD2 group at position at position 3 and the terminal CD3 group are almost the same in both phases. These results are interpreted in terms of the effects of hydrocarbon chain organization on the vibrational modes.

Protein-lipid interactions. High-field deuterium and phosphorus nuclear magnetic resonance spectroscopic investigation of the cytochrome oxidase-phospholipid interaction and the effects of cholate

Deuterium quadrupole-echo Fourier transform nuclear magnetic resonance spectra (at 34 MHz) and phosphorus-3 1 Fourier transform nuclear magnetic resonance spectra (at 60.7 MHz) have been obtained of 1-(6,6-dideuteriopalmitoyl)-2-oleyl-sn-glycero-3-phosphocholine dispersions in excess water in the absence of, and complexed with, the membrane enzyme cytochrome oxidase (cytochrome c:02 oxidoreductase, EC 1.9.3.1). Thereby, we have investigated the effects of the detergent sodium cholate, and of temperature, on protein-lipid interactions in this system. Our results strongly suggest that residual detergent in these protein-lipid complexes causes a significant disordering of hydrocarbon chain and head group organization as determined by deuterium quadrupole splittings (AvQ) and phosphorus chemical shielding anisotropies (Aa). At low (-2 wt %) cholate levels, C6 AuQ and 31P ha values in the protein-lipid complexes (containing T h e nature of the interactions between proteins and lipids in biological membranes is a topic of considerable current t From the Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (D.M.R. and E.O.), and the Department of Chemistry and the Laboratory for Bioenergetics, State University of New York at Albany, Albany, New York 12222 (J.C.H. and T.E.K.). Received April I I , 1979; revised manuscript received Augusl 15, 1979. This work was supported by the National Institutes of Health (Grant HL-19481 to E.O. and Grants GM-16767 and HL12576 to T.E.K.) and in part by the Alfred P. Sloan Foundation (E.O.). *Correspondence should be addressed to this author. He is an Alfred P. Sloan Research Fellow, 1978-1980. *Supported in part by a National Institutes of Health, Cell and Molecular Biology Training grant (Grant GM-07283). -70 wt % protein) are only about 8% smaller than in pure phospholipid bilayers at the same temperature, between 20 and 35 “C, suggesting rather similar structural organization. There are, however, significant line-width increases, especially on approaching T, in both deuterium and phosphorus spectra, indicating increased correlation times, in the region of the membrane surface. These results obtained on protein-lipid complexes containing an unsaturated phospholipid are in agreement with those of a previous study utilizing a disaturated phospholipid [Kang, S . Y., Gutowsky, H. S . , Hsung, J. C., Jacobs, R., King, T. E., Rice, D., & Oldfield, E. (1979) Biochemistry 18, 3257-32671 but differ significantly from those of a similar study employing 1-(5,5-dideuteriopalmitoyl)-2-oleyl-sn-glycero-3-phosphocholine [Seelig, A,, & Seelig, J . (1978) Hoppe-Seyler’s 2. Physiol. Chem. 359, 1747-17561. interest due to the frequent observation that a variety of membrane enzymes appear to require lipids in order to express their full biological activity [e.g., Yu et al. (1975), Gennis & Jonas (1977), Tanford & Reynolds (1976), Bennett et al. (1978), and Vik & Capaldi (1977)l. The most frequent model for the nature of the protein-lipid interaction is that the lipids are somehow immobilized (Jost et al., 1973a,b, 1977; Marsh et al., 1978; Warren et al., 1974, 1975; Longmuir et al., 1977) by the rigid protein “wall” (Moore et al., 1978), a cholesterol-like effect (Hong & Hubbell, 1972; Kleemann & McConnell, 1976), and that as a result the lipid hydrocarbon chain organization becomes far more restricted, leading to increased molecular order parameters (Dahlquist et al., 1977; MarEelja, 1976; Jost & Griffith, 1978; Scott & Cherng, 1978). 0006-2960/79/0418-5885S01 .OO/O , 1

Physical studies of cell surface and cell membrane structure: Deuterium nuclear magnetic resonance investigation of deuterium-labelled N-hexadecanoylgalactosylceramides (Cerebrosides)

1. 1. Deuterium Fourier transform nuclear magnetic resonance spectra of a series of N-palmitoylgalactosylceramides (cerebrosides) specifically labelled with deuterium at one of positions 2′, 6′, 10′ and 16′ of the acyl chain, or in the C-6 hydroxymethyl group of the galactose residue, have been obtained using a spin-echo technique at 34.1 MHz with a homebuilt superconducting magnet spectrometer. 2. 2. The effects of temperature and cholesterol on the deuterium spectra have been investigated. The results indicate, when compared at the same reduced temperature, that the hydrocarbon chain organization in the liquid crystalline phase of palmitoylgalactosylceramide is essentially identical to that seen in similar chain length glycerophospholipids. In particular, two sets of quadrupole splittings are seen for a 2′-labelled N-palmitoylgalactosylceramide, indicating non-equivalent deuterons as noted previously for phospholipids. 3. 3. Two sets of quadrupole splittings are observed for the headgroup C-6-labelled N-palmitoylgalactosylceramide. It is proposed that these signals arise from the enantiomeric R and S lipids, and that motion of the hydroxymethyl group is slow (greater than 10 −5 s). These results suggest the presence of a hydrogen bond network in the polar headgroup region.

The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayers was investigated by X-ray diffraction methods. Electron density profiles at 5 Å resolution along with chain tilt and chain packing parameters were obtained and compared for phosphatidylcholine/cholesterol bilayers and for pure phosphatidylcholine bilayers in both the gel and liquid crystalline states. The cholesterol in the bilayer was localized by noting the position of discrete elevations in the electron density profiles. Cholesterol can either increase or decrease the width of the bilayer depending on the physical state and chain length of the lipid before the introduction of cholesterol. For saturated phosphatidylcholines containing 12–16 carbons per chain, cholesterol increases the width of the bilayer as it removes the chain tilt from gel state lipids or increases the trans conformations of the chains for liquid crystalline lipids. However, cholesterol reduces the width of 18 carbon chain bilayers below the phase transition temperature as the long phospholipid chains must deform or kink to accomodate the significantly shorter cholesterol molecule. Although cholesterol has a marked effect on hydrocarbon chain organization, it was found that, within the resolution limits of the data, the phosphatidylcholine head group conformation is unchanged by the addition of cholesterol to the bilayer. The head group is oriented parallel to the plane of the bilayer for phosphatidylcholine in the gel and liquid crystalline states and this orientation is not changed by the addition of cholesterol.